Apparatus for the continuous preparation of crystalline substances

ABSTRACT

EQUIPMENT FOR CONTINUOUSLY PRECIPITATING CRYSTALLINE SUBSTANCE OF PREDETERMINED DIMENSIONAL REGULARITY, QUALITY AND QUANTITY INCLUDING A REACTOR HAVING A PORTION INCREASING IN DIAMETER FROM BOTTOM TO TOP WITH A TUBE ENTERING FROM THE TOP AND EXITING INTO THE REACTOR NEAR THE BOTTOM THEREOF, THE TUBE INCREASING IN DIAMETER NEAR THE LOWER PART OF THE REACTOR, AND A SECOND TUBE ENTERING NEAR THE BOTTOM AND EXTENDING UPWARDLY ALONG AN AXIS WHICH IS IN ALIGNMENT WITH THE FIRST TUBE AND EXITING AT A POINT BELOW THE END THEREOF, SO THAT A MAIN STREAM OF MOTHER LIQUOR CAN BE INTRODUCED THROUGH THE FIRST TUBE IN A DOWNWARDLY DIRECTION AND A SECONDARY STREAM OF MOTHER LIQUOR CAN BE INTRODUCED THE SECOND TUBE TO YIELD A ZONE OF SUBSTANTIALLY NO SPEED AT THE LOWER END OF THE REACTOR.   D R A W I N G

April 13, 1971 QLIVIER ETAL 3,574,564

. APPARATUS FOR THE CONTINUOUS PREPARATION OF CRYSTALLINE SUBSTANCESOriginal Filed Jan. 19, 1967 I 2 Sheets-Sheet 1 Fig-I I I v tlyiliid 20J 23 Z9 Mum} 70/6 5% MW} April 13, 1971 H, QLMER ET AL 3.514564APPARATUS FOR THE CONTINUOUS PREPARA'llON OF CRYSTALLINE SUBSTANCES 7Original Filed Jan. 19, 1967 2 Sheets-Sheet 2 United States Patent Int.Cl. 1301a 9/00 U.S. Cl. 23273 Claims ABSTRACT OF THE DISCLOSUREEquipment for continuously precipitating crystalline substance ofpredetermined dimensional regularity, quality and quantity including areactor having a portion increasing in diameter from bottom to top witha tube entering from the top and exiting into the reactor near thebottom thereof, the tube increasing in diameter near the lower part ofthe reactor, and a second tube entering near the bottom and extendingupwardly along an axis which is in alignment with the first tube andexiting at a point below the end thereof, so that a main stream ofmother liquor can be introduced through the first tube in a downwardlydirection and a secondary stream of mother liquor can be introducedthrough the second tube to yield a zone of substantially no speed at thelower end of the reactor.

This application is a division of co-pending application Ser. No.610,267 filed Jan. 19, 1967.

The present invention relates to apparatus for the continuouspreparation of crystalline substances of mineral or organic nature. Itenables the crystallized product to be obtained as particles of regularand determined dimensions by a direct and continuous preparation method.

Crystallization arrangements have already been described which aredirected to continuously obtaining regular crystals by cooling orpartial evaporation of supersaturated solutions. In these arrangements,the crystals increase in size in suspension in a current circulatingalong a closed circuit, into which the product is introduced in thedissolved state and is extracted continuously or periodically in thesolid state.

On the other hand, arrangements for the preparation of a mineralsubstance, in which a bed of particles is maintained in suspension by acooled ascending current, are already known, particularly in accordancewith British Pat. No. 911,664 of the Apr. 30, 1958, in the name ofLaporte Chemicals Limited. This ascending current of super-staturatedsolution circulates in a vertical column, of which the upper portion hasa progressively increasing section and in which the speed of circulationof the said current is reduced. In accordance with this process, thesuper-saturation of the ascending liquor current is produced in the samebed of particles.

French Pat. No. 1,187,352 of the Nov. 29, 1957, in the name of SocietedEIectrOchimie, dElectrometallurgie et des Acieries Electriques dUgine,also describes a method for the continuous formation of aglomerates ofdetermined dimensions from fine crystals, according to which singlecrystals are united by bringing then into suspension in a supersaturatedsolution of the same substance while subjected to continuous movement.

Apparatus for the preparation of crystalline substances has beendeveloped according to the present invention,

3,574,564 Patented Apr. 13, 1971 which has the advantages of obtaining afinal product with a remarkable predetermined dimensional regularity,quality and quantity, by continuous production and while avoiding anyclogging and any solid deposition on the walls of the apparatus.

This apparatus difiers from known apparatus due to the fact that it usesthe principle of the ascending current at decreasing speed not forobtaining a product crystallized by cooling or evaporating a solution ofthis product, but for obtaining a solid product resulting from thechemical reaction of reactants used in the form of solutions. On theother hand, it is distinguished therefrom by the fact that it does notuse any continuous or intermittent supply of a preformed product. Theprecipitation results initially from solutions simply being brought intocontact in the apparatus itself, the reaction of said solutionsproducing the expected crystalline product.

This apparatus permits crystalline substance to be continuously obtainedby direct precipitation from at least two chemical compounds reactingwith one another and used in the form of solutions, in which a bed ofparticles is supported by an ascending current of mother liquor, ischaracterized by the following points, taken separately or incombination:

(a) A current of mother liquor flows through a closed main circuit,comprising in succession a descending portion and an ascending portion,separated by a zone with a substantially zero circulation speed;

(b) The compounds of which the reaction produces the final product byprecipitation are introduced in solution into this current, outside theascending portion of the closed circuit;

(c) One of the reactants is introduced into the descending portion ofthe circuit in the vicinity of the opening of this descending portioninto the zone where the speed of circulation is substantially zero;

(d) The mother liquor is super-saturated outside the bed of particles;

(e) A secondary current of mother liquor is established in a directionsuch that it tends to cancel out the speed of the main current and toestablish a zone where the speed of circulation is practically zero;

(f) The secondary current is introduced in an ascending direction;

The secondary current is introduced in a direction which is almosthorizontal;

(h) The desired grain size is obtained by regulating the rate of flow ofthe mother liquor in the descending portion of the main current;

(i) The super-saturation of the mother liquor is increased by cooling,in a zone externally of the descending portion, ascending portion andthe zone of substantially zero speed, as described above;

(j) The precipitation of the final product is etfected essentially inthe substantially zero speed zone and in the ascending portion of thecircuit.

This apparatus is suitable for the preparation of numerous mineral ororganic crystalline substances, such as sodium, potassium and ammoniumpersulphates, and particularly sodium perborate.

When the present invention is used in connection with sodium perborate,it has for its object to provide apparatus for conducting a continuousprocess, according to which a bed of particles is supported in thereactor by an ascending current of mother liquor, the hydrogen peroxideand the sodium metaborate being introduced into the circulating motherliquor outside the bed of particles; the metaborate can also be preparedin situ, for example from borax.

The mother liquor flows in a closed circuit. Drawn in at the top of thereactor, it is saturated outside the latter,

first by cooling in the exchanger and secondly by introduction ofstoichiometric quantities of hydrogen peroxide, sodium metaborate, whichlatter may optionally be in excess, and stabilizer.

The super-saturated liquid is assured a circulation such that nocrystallization is produced before it is brought into contact with thebed of particles.

The major part of the mother liquor, forming the main current, isintroduced into the lower part of the reactor through a tube extendingfrom top to bottom of the apparatus. The remaining portion of the motherliquor, forming the secondary current, enters in counter-current throughthe base of the reactor and is directed in such a way as to create azone in which the speed of circulation is substantially zero. Thepurpose of this counter-current, withdrawn from the line of circulationof the mother liquor, is to facilitate the deposition of perborateparticles which have reached the desired size, this being by reducingthe eddy caused at the exit of the main current.

The same effect of regular depositions of crystalline particles isobtained by replacing this secondary current by a fixed deflector,centered on the preferably divergent discharge of the main current, orby causing it to discharge horizontally through slots formed laterallyin the tube by which the said secondary current is introduced.

The control of the rate of flow of the mother liquor in the circulationline permits of obtaining the desired grain size for the particlessituated in the lowest part of the bed. The mean diameter of theparticles is connected with the minimum rate of flow necessary for thefiuidation thereof.

For a given rate of flow of mother liquor, it is also possible to act onthe grain size by the height of the apparatus, and thus by the residencetime of the mother liquor in the reactor. It is thus possible to obtaina very high percentage of particles which are between two similardimensions, the value of these latter being a function of the rate ofcirculation.

The reactor preferably has a section which increases in an upwarddirection.

This conical form of the reactor is designed to ensure a speed gradientfor a .given rate of flow of the. ascending liquor. As the speeddecreases in an upwarddirection, the result is an automatic grading ofthe particles: those having the desired dimension fall to the bottom ofthe reactor, where the product is extracted, the other particles beingin dynamic equilibrium at a level which is higher as the particles aresmaller, and descending in proportion with their growth.

The height of the bed of particles is only limited by the dangers of theapparatus becoming clogged when the quantity of grains in the column istoo large and interrupts the circulation of the mother liquor.

In order to counteract the soiling of the walls, which is more to befeared in the external circulation line than in the reactor itself, itis possible to visualize two lines per apparatus, one being in operationwhile the other is being cleaned by reheating.

The particles which fall as they increase in size are liable to bedeposited on the walls; in order only to obtain grains of the desireddimensions, it is also advantageous to return to the reaction medium theperborate which has not reached the desired grain size.

For this purpose, the walls are constantly swept over by a scrapersystem. This scraping arrangement can be formed by brushes, fixed ondescending tubes bearing on the walls by means of adjustable resilienthoops. These brushes are of a flexible plastic material, such asflexible polyvinyl chloride, polytetrafluoroethylene, etc. The upperpart of each tube is fast with a driving ring which rotates at the topof the apparatus and communicates its movement to the entire cleaningassembly. This arrangement thus ensures the cleanliness of the walls atthe same 4 time as the resuspension of the perborate grains which havenot reached the desired size.

One installation for the continuous manufacture of crystallinesubstance, particularly sodium perborate, in accordance with theinvention, will hereinafter be described by way of example and isillustrated on the accompanying drawings.

According to FIG. 1 of the accompanying drawing, the installationnecessary for carrying the invention into effect comprises the reactor,inside which is effected the ascending circulation of the mother liquorand the descending movement of the crystals obtained by reaction andcrystallization, and the circulation outside this reactor.

This reactor comprises three parts. The upwardly flaring truncated base1 is the zone where the speed of the liquid is zero, the solid productbeing deposited therein when it reaches the desired size. The minor baseof the cone forms the bottom of the reactor 2 and permits the extractionof the perborate by way of one or more valves 3 which have direct flowand are of the quick-opening type. A single orifice of small section mayserve the same purpose when it is constantly open.

Arranged above the truncated base 1 is a part 4 of constant section. Inthis zone of the reactor, the very super-saturated mother liquor is incontact with fairly large crystals. The strong super-saturation of themother liquors also creates a liquid-solid exchange potential sufficientfor bringing the grains to the required dimension. When this is reached,the crystalline product is deposited in the first part of the reactor.

The upper part of the truncated reactor 6 is flared upwardly to asection such that no appreciable nucleus of the solid product is anylonger in suspension. Situated at this height is the intake 7 of thecirculation pump 8. A height of liquid 9 sufiicient to avoid anyturbulence is maintained above this intake.

This part 6 of the reactor ensures an exhaustion of the mother liquorswhich arrive in the upper zone close to saturation point, at the sametime as a grading of the crystalline particles as a function of theirsize. It is possible to reduce the total height of the reactor byadopting several conicities calculated in such a way as to avoid anyeddies. As a modification, the reactor can thus be made conical from thebottom.

In this reactor, a permanent exchange of material is assured between themother liquors, which are desaturated as they ascend, and thecrystalline particles, which increase in size as they descend. Thecrystalline particles are kept in suspension by the flow of motherliquor which is accurately regulated.

The circulation outside the reactor carries along the just saturatedmother liquor drawn in at 7 by means of the valve 11 and the circuit 12and saturated with a reactant, the metaborate, introduced at 13 into thecircuit 12.

The mother solution is then brought under pressure by the pump 8, whichdelivers two currents by means of the valves 14 and 15. The main currentis reintroduced, after cooling in the exchanger 16, to the base of thereactor through the tube 5 which descends in the center of the reactor.The descending speed .in this tube is very much higher than theascending speed in the reactor.

The tube 5 ends in a divergent portion 17, into which the secondreactant, the hydrogen peroxide, is introduced at 18.

The secondary current, of which the rate of flow is smaller than that ofthe main current, is introduced, after passing through the valve 15 andthe conduit 19, through the vertical tube 20 extending to the reactorthrough the bottom. In order to give the secondary current, on enteringthe reactor, a general direction close to the horizontal, and as can beseen from FIG. 2 of the accompanying drawing, which shows the details ofthe base of the reactor, the liquid discharges from the tube 20 througha series of slots 25 formed laterally in the cylindrical wall of theintroduction tube. The main current being thus deflected, the result isthe establishment of a zone in which the speed of circulation of theliquid is very slow, this facilitating the deposition of the finishedproduct.

The cleaning of the internal face of the reactor by a scraper deviceapplied resiliently to the walls does not cause any disturbance in thecrystallization phenomenon, due to a size reduced to a minimum in thereaction medium and a slow speed of rotation. This speed is of the orderof a few revolutions per hour, in the case of sodium perborate.

An upper ring 21 carries at least three tubes 22 held bearing againstthe walls by resilient hoops 23. Fixed on these tubes are brushes whichrub on the wall.

FIG. 3 of the accompanying drawing, which represents a plan view of theWall-scraper arrangement, shows the position of these differentelements.

In a pilot apparatus in accordance with the accompanying drawings, it ispossible to obtain an hourly production of 15 kg. of sodium perborate bysupplying the apparatus with the corresponding quantities of hydrogenperoxide and sodium metaborate in a 200 g./1. solution, it beingpossible for this latter reactant to be introduced in an excess of Thehydrogen peroxide employed is 70% by weight. The process provides thepossibility of using the hydrogen peroxide in any concentration,provided that the solid content of the mixture of mother liquor andperborate extract is regulated so as to extract at the same time thevolume of water of dilution of the reactants necessary for itsproduction.

In this example, the rate of flow of the mother liquor in the reactor isestablished at 75 litres per minute, this corresponding to a speed ofascent of 2.88 cm. per second in the part of constant section, with aresidence time of the perborate in the reactor regulated to be one hour.Crystalline products are obtained, of which the density and the grainsize are set out in the following Table I:

TABLE I.-PERCENT OF REFUSE COLLECTED ON SCREENS WITH THE FOLLOWING MESHOPENINGS (AFTER OPERATING FOR 4 HOURS) Under the above operatingconditions, a crude sodium perborate is obtained with a crystallizationand grain size as regular as that given in Table I, that is to say,having an accumulated refuse of 60% between the screens of opening 590aand 351a.

Using a reactor in which the cylindrical part has a length twice thatindicated in FIG. 1 of the accompanying drawing, the rate of flow ofmother liquor in the reactor being fixed so as to obtain a speed ofascent of 3.5 cm. per second in the constant section zone, and theresidence time being adjusted to 2 hours, a product is obtained of whichthe grain size anasysis is set out in Table II.

TABLE IL-P-ERCENT OF REFUSE ACOUMULA'I'ED ON SCREENS TABLE IL-ContinuedMesh opening (,u): Percent Under the above operating conditions, a crudemedium perborate is obtained with regular particles and a perfectlysatisfactory grain size as corresponds to Table II, that is to say,giving 74% of refuse between the screens of opening 1168a and 833,1.

What we claim is:

1. Apparatus for the continuous direct precipitation of crystallinesubstance of predetermined dimensions comprising a reactor having asection increasing from bottom to top, a first tube entering the reactorat the top thereof and extending from the top of the reactor and exitinginto the reactor at a point near the bottom thereof, said first tubeincreasing progressively in diameter near the lower part of the reactorand including means for introducing one of the reactants into thereactor at the exit of said first tube, a second tube entering thereactor near the bottom thereof and extending upwardly along an axiswhich is in alignment with the first tube and exiting at a point belowthe end of said first tube, and in proximity thereto means to introducea main stream of mother liquor through said first tube in a downwardlydirection and means to introduce a secondary stream of mother liquorthrough said second tube to yield a zone of substantially zero speed atthe lower end of said reactor.

2. Apparatus according to claim 1, wherein the second tube is providedwith means to cause the secondary stream to discharge in a horizontaldirection in the lower end of said reactor.

3. Apparatus according to claim 1, wherein the second tube is closed atits upper end and slots are provided 1at erally in the tube fordischarging the secondary stream in a horizontal direction in the lowerend of said reactor.

4. Apparatus according to claim 1, wherein the reactor is provided withan internal wall scraping system.

5. Apparatus according to claim 4, wherein the scraping system is formedof brushes fixed on descending tubes bearing on the inner wall of thereactor by means of adjustable resilient hoops, the upper portions ofthe descending tubes being fixed to a driving ring rotating at the topof the apparatus.

References Cited UNITED STATES PATENTS 351,651 10/1886 Porter 159-452,318,463 5/ 1943 Bussrnan 23-273 2,424,207 7/ 1947 Otto 23-2732,764,488 9/1956 Slattery 23-273 3,068,077 12/1962 Witte 23-2733,459,509 8/1969 Aoyama 23-273 FOREIGN PATENTS 973,936 7/ 1960 Germany23-273 NORMAN YUDKOFF, Primary Examiner S. SI LVERBERG, AssistantExaminer US. Cl. X.R. 23-301; 159-45

